There is no useful universal size
The same letter height can perform very differently depending on the font, stroke width, contrast, illumination and viewing angle. A sign read by pedestrians at ten metres has a different task from a pylon sign read by drivers moving through an intersection.
Evaluate real sightlines
Measure the normal viewing distance and photograph the proposed location using a standard lens. Note the amount of time the viewer can see the sign and whether trees, parked vehicles or neighbouring signs interrupt the view.
- Viewing distance and approach angle
- Expected pedestrian or vehicle speed
- Uppercase and lowercase letter shapes
- Day and night contrast
- Number of words in the message
Shorter messages can be larger
When the available sign area is fixed, every extra word reduces the size of the primary identification. Keep the building sign focused on the business name. Services, phone numbers and promotional details usually belong on windows, digital displays or other supporting surfaces.
Review a scaled elevation rather than judging letters on a blank artboard. The surrounding facade shows whether the sign has enough breathing room and contrast. For critical roadside locations, a temporary full-size paper or projected test can help confirm visibility from the actual approach before fabrication begins.